This invention relates to injection molding and more particularly to a single nozzle sprue gated injection molding system wherein there is reduced heat loss from the heated nozzle to the surrounding mold.
In this type of system, the heated nozzle is seated in a well in the cooled mold. As shown for instance, in the applicant's U.S. Pat. No. 4,557,685 entitled "Heated nozzle for Injection Molding Apparatus" which issued December 10, 1985, the nozzle has an insulation bushing which seats against a shoulder to locate the nozzle with an insulative air space between it and the surrounding mold, the applicant's U.S. Pat. No. 4,238,671 entitled "Spruce Bushing with Cast in Heater Element" which issued Dec. 9, 1980 shows the nozzle being held in place by pressure from the molding machine nozzle. However, it is preferable to hold the nozzle in place in the well using a retaining collar bolted to the mold as shown in FIG. 15 of Mold-Masters Limited brochure A-MS-058-1182 entitled "Masters-Shot Nozzles" dated November 1982.
As is well know, it is critical to the successful operation of the system that the temperature of the melt be maintained within a predetermined narrow temperature range as it flows through the nozzle. However, it has been found that the previous nozzles of this type suffer greater temperature drop at the end than in the middle which makes a uniform temperature difficult to achieve. One attempt to deal with this problem is to increase the pitch of the helical heating element in the middle as shown, for instance, in the applicant's Canadian patent application Ser. No. 549,519 filed Oct. 16, 1987 entitled "Injection Molding Nozzle with Resilient Sealing and Locating Flange." Canadian application Ser. No. 549,519 also discloses a sealing and locating flange which extends from the nozzle across the insulative air space and abuts against the surrounding inner surface of the well. As will be appreciated, each of the insulation bushing, retaining collar and sealing and locating flange which accurately locate the nozzle and hold it in place necessarily result in some heat loss from the area which compounds the uniform temperature problem.
The applicant's Canadian patent application Ser. No. 520,519 filed Oct. 15, 1986 entitled "Sealing and Retaining Bushing for Injection Molding" shows a bushing with a flanged outer portion which extends between the manifold and the back plate to hold the manifold in place. However, this is a multi-cavity valve gated system and the bushing also seals against leakage around the reciprocating valve pin which extends through it.